The present invention relates to solid-state imaging devices capable of removing undesired influences by false signals resulting in smear and blooming phenomena, and more particularly, relates to solid-state imaging device such as a charge coupled device (CCD) capable of excluding undesired influences caused by false signals even when signal charges having a low level are transferred. Here, a smear phenomenon means that white points are extended in vertical stripes in a reproduced picture plane, which are caused by light impinging onto signal lines and the CCD, or signal charges occurring inside a semiconductor substrate, which extend by diffusion and affect adjacent pixels and a transfer portion. Blooming is a phenomenon which resembles smear phenomenon and often accompanies a smear. Blooming is a phenomenon in which a white portion is extended around a light in the manner that a white flower is blooming on the picture plane, because an incident strong light makes the pixels saturated and signal charges overflow to affect the adjacent pixels, signal lines and vertical transfer CCD.
There is described an example of a conventional solid-state imaging device with reference to attached drawing. FIG. l is a sectional view showing a sensitive portion of an interline transfer (IT) type solid-state imaging device, in which a p-type impurity well 2 is formed on a surface of an n-type semiconductor substrate 1. The p-well 2 has a recessed photodiode 3 for generating signal charges corresponding to an incident light amount, and a vertical transfer channel 4 for transferring the signal charges in the vertical direction on the paper. A metal electrode 8 is formed on the vertical transfer channel 4 through an insulating layer such as a silicon oxide layer (not shown). The metal electrode 8 functions in part as a gate for transferring the signal charges accumulated in the photodiode 3 to the vertical transfer channel 4 by adding a high electric field, and in part for delivering the signal charges in the vertical transfer channel 4 in one direction by adding a moving electric field of a plurality of electrodes. The photodiode 3 has a construction in which a p.sup.+ -type impurity layer is provided on an upper surface of a semiconductor device in order to reduce a dark current occurring on the upper surface of the semiconductor device. A p-type barrier well 5 is formed around the transfer channel 4 for preventing the transfer channel 4 from an invasion of smear charges. The p-type barrier well 5 has an impurity density of one digit higher than the p-well 2 in order to suppress an extension of a depletion layer in the transfer channel 4. Furthermore, an element separation layer 6 is formed by a p-type impurity and adjacent to the transfer channel 4. A description of an insulation layer and a protective layer is omitted.
There are two types of solid-state imaging devices, namely, an IT type and a frame interline transfer (FIT) type. The IT type has a sensitive portion in which a photodiode and transfer portion are horizontally arranged to one another, while the FIT type has accumulating portion formed from the transfer portion in addition to the sensitive portion. In the IT type solid-state imaging device, the signal charges are transferred from the photodiode to the adjacent transfer portion during a vertical retrace line interval of a television signal. On the contrary, in the FIT type solid-state the signal charges are moved from the photodiode to the adjacent transfer portion, and at the same time, the signal charges are transferred to the accumulating portion. Therefore, since the moving distance of the signal charges in FIT type devices is longer than in the IT type device, it is necessary to move the charges very quickly in the transfer channel.
However, when the solid-state imaging device has the construction including the barrier well 5 effective to prevent the smear phenomenon, it is possible to prevent the extension of the depletion layer in the vertical transfer channel 4 into the semiconductor device, thereby decreasing the transfer efficiency by reducing a fringe electric field added to the transfer channel in a bulk. In the FIT type device for moving the signal charges quickly, when the barrier well is provided, there occur problems that the vertical resolution is reduced at transferring low level signal charges. Accordingly, the barrier well is not provided for the FIT type device to keep the high-speed operation of the vertical transfer of the signal charges to increase the fringe electric field in the bulk. Alternatively, the FIT type device is used in a range for obtaining the necessary transfer efficiency by decreasing the transfer frequency at the sacrifice of the high-speed operation.